Synergistic Toxicity Produced by Mixtures of Biocompatible Gold

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Synergistic Toxicity Produced by Mixtures of Biocompatible Gold Nanoparticles and Widely Used Surfactants Aurora L. Ginzburg, Lisa Truong, Robert L. Tanguay, and James E. Hutchison ACS Nano, Just Accepted Manuscript • Publication Date (Web): 26 Apr 2018 Downloaded from http://pubs.acs.org on April 26, 2018

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Synergistic Toxicity Produced by Mixtures of Biocompatible Gold Nanoparticles and Widely Used Surfactants Aurora L. Ginzburga, Lisa Truongb, Robert L. Tanguayb*, James E. Hutchisona* a

Department of Chemistry and Biochemistry, University of Oregon, Eugene, Oregon 97403,

United States,

b

Department of Environmental and Molecular Toxicology and the Sinnhuber

Aquatic Research Laboratory, Oregon State University, Corvallis, Oregon 97333, United States. Corresponding authors: [email protected] and [email protected]

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Abstract Nanoparticle safety is usually determined using solutions of individual particles that are free of additives. However, the size-dependent properties of nanoparticles can be readily altered through interactions with other components in a mixture. In applications, nanoparticles are commonly combined with surfactants or other additives to increase dispersion or to enhance product performance. Surfactants might also influence the biological activity of nanoparticles; however, little is known about such effects. We investigated the influence of surfactants on nanoparticle biocompatibility by studying mixtures of ligand-stabilized gold nanoparticles and Polysorbate 20 in embryonic zebrafish. These mixtures produced synergistic toxicity at concentrations where the individual components were benign. We examined the structural basis for this synergy using solution-phase analytical techniques. Spectroscopic and X-ray scattering studies suggest that the Polysorbate 20 does not affect the nanoparticle core structure. DOSY NMR showed that the hydrodynamic size of the nanoparticles increased, suggesting that Polysorbate 20 assembles on the nanoparticle surfaces. Mass spectrometry showed that these assemblies have both increased uptake and increased toxicity in zebrafish, as compared to the gold nanoparticles alone. We probed the generality of this synergy by performing toxicity assays with two other common surfactants, Polysorbate 80 and sodium dodecyl sulfate. These surfactants also caused synergistic toxicity, although the extent and timeframe of the response depends upon the surfactant structure. These results demonstrate a need for additional, foundational studies to understand the effects of surfactants on nanoparticle biocompatibility and challenge traditional models of nanoparticle safety where the matrix is assumed to have only additive effects on nanoparticle toxicity. Keywords: nanotoxicology, mixture toxicity, mixture effects, zebrafish, polysorbate, synergy.


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Inorganic nanoparticles (NPs) have emerged as promising materials with properties that advance a wide range of applications including: electronic displays,1 water purification,2 cosmetics,3 textiles,4 medical imaging,5 and drug delivery.6 Despite their enhanced properties, NPs are rarely used as individual components within applications. NP-containing products and technologies often combine NPs with molecular species, either by chemically linking the two (e.g., a hybrid nanostructure)7,8 or by mixing them freely in solution.9,10 Examples of these approaches include mixing UV-scattering NPs with lithium stearate in sunscreen to help NPs adhere to skin,11 and using tethering ligands to link silver NPs to clothing to yield an antibacterial coating that is durable through repeated laundering.8 Although the use of mixtures to enhance nanomaterial performance has been demonstrated, it remains unclear how the molecular components of these mixtures will influence any health or environmental impacts of the NPs. Thus, understanding the effects of NPs in combination with other species is important to avoid negative impacts and to design safer materials. Studies on mixture toxicity are important complements to studies of individual compounds because chemicals can influence one another’s toxicity.12,13 When a mixture is more toxic than the combination of individual components the interaction is classified as synergistic. Historically, synergism was thought to be rare, and considered negligible for low-dose mixtures typically encountered in nature.14 While this is generally true for the classically studied pesticide mixtures, recent reviews of the literature suggest that mixtures in other environmentally relevant contexts, like biocide formulations, have a much higher incidence of synergy (26% incidence reported in tested biocide mixtures in contrast to 7% of pesticide mixtures).13 Synergistic toxicities have been observed between mixtures of mycotoxins at compositions and concentrations typically found in foods,15 and between metals and polycyclic aromatic


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hydrocarbons, which are ubiquitous contaminants in both freshwater and coastal environments.16 The European Union has reviewed concerns that current risk assessments may be inadequate for predicting outcomes of exposures to mixtures and, thus, have prioritized further studies on mixture toxicity.17,18 Traditional assessments of mixture hazards are based on data collected on the individual components, as opposed to measurements of the mixtures. When the components are expected to have different modes of action, an Independent Action model considers the likelihood of each independent chemical causing mortality. On the other hand, a Concentration Addition model is used when components have similar modes of action. In this approach chemicals are treated as additive and the impacts are summed together to assess mixture hazards.3,19 When these models work, they are convenient because they decrease the time and expense of testing the toxicity of a staggering number of possible combinations and concentrations of components in mixtures.20 Unfortunately, these convenient models can be misleading because they assume that no chemical interactions and/or reactions occur, both of which can lead to synergistic or antagonistic effects.12 Mixture synergies are especially relevant in the case of NPs because their surface properties increase their propensity for interacting with other species.21–23 NPs often have catalytic activity and high surface area that maximize their interactions with molecules.24 Even when solution components are not covalently bound or undergoing chemical transformations at the NP surface, they are often readily adsorbed onto the ligand shell. Examples include protein corona formation when NPs are immersed in biological media, and the acquisition of natural organic matter coatings when NPs enter environmental systems.23,25–27 Such interactions compromise the application of some general axioms of NP hazard, such as smaller NPs tend to be more lethal than larger ones per mass-based dose21 and uncoated toxic metal cores often leach toxic ions.28


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The addition of additives can alter NP biological reactivity, toxicity, stratum corneum penetration depth, and biodistribution, thereby making it difficult to predict NP safety in mixtures.27,29 To date, little is known about how mixture-induced changes to NPs (notably ones that do not alter NP dispersibility, core size, or charge) will affect their toxicity. We recently observed that a series of gold nanoparticles (AuNPs) possessing a ligand shell known to produce some of the safest AuNPs30,31 exhibits up to 83% increased toxicity when dispensed with a digital dispensing system rather than standard manual hand pipetting. The digital dispensing system, a Hewlett-Packard D300 (referred to as the BioPrinter), uses inkjet printing technology to precisely dispense picoliter-sized droplets of materials for biological assays. The only substantive difference between this method of dispensing and hand pipetting is the presence of a small amount of surfactant (Polysorbate 20) used to control surface tension within the narrow channels in the BioPrinter printhead. The fact that this non-toxic surfactant,32 often used as a food additive,33 dramatically affected the toxicity of the AuNPs, led us to question how the surfactant influences the NP toxicity and whether common surfactants might alter the toxicity of AuNPs. Although surfactants have been widely used to disperse nanoparticles,11,34 and are even used as stabilizing ligand shells on NP surfaces, studies that investigate how free surfactant molecules affect NP properties are sparse. In fact, a review of the literature shows that only a few studies have investigated the effects of mixture components on NP toxicity. The work that has been done is largely focused on how the toxicity of zinc oxide NPs is influenced by components encountered in food and during digestion, such as vitamin C9 or fatty acids.35 Herein, we combined toxicological assays with detailed structural analysis to develop an understanding of how additives influence NP toxicity. We studied the biological effects of 5 ACS Paragon Plus Environment

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polyethylene glycol-stabilized gold nanoparticles (a class of NPs known to be some of the least toxic),31 in the presence of a non-ionic, nontoxic32 surfactant, Polysorbate 20 (PS20); chemical structures are shown in Figure 1. Biological assays performed using embryonic zebrafish showed that mixtures of these two chemicals result in a synergistic toxicity. We the nanoparticles, Figure 1. Structure of mercaptoethoxyethoxyethanol (MEEE) stabilized gold nanoparticles (AuNPs), and primary surfactant, Polysorbate 20 (PS20), studied in this work.

examined the structural basis for this synergy using a combination of 2D-NMR, small-angle

X-ray scattering, and UV-vis. We found that the surfactant assembles on the NP surface, effectively increasing the hydrodynamic particle size, but does not disrupt the metallic core. Mass spectrometry measurements show that PS20 increases the biological uptake of the AuNP/PS20 assemblies. However, the higher quantity of gold accumulated within the zebrafish embryos (i.e., tissue burden), alone, does not account for the increased toxicity. These results suggest that the assembled structures themselves are more toxic. Two other common surfactants were investigated, and both increased the NP toxicity, although to varying extents and on different timeframes. Results and Discussion In the initial BioPrinter experiments, we observed that even a small amount (0.003% v/v) of PS20 yielded an AuNP mixture with synergistic toxicity. This result was surprising because both the PS20 and AuNPs showed statistically negligible background mortality when tested individually at these concentrations, and synergistic effects are thought to be essentially nonexistent at the concentrations we were studying (low µg/mL).20,36 Because our preliminary 6 ACS Paragon Plus Environment

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results suggested that NP toxicity may be susceptible to mixture effects in ways not yet reported, we aimed to investigate (i) at what concentrations do these effects occur, (ii) what is the structural basis for the synergy, and (iii) can these effects be avoided through the use of alternative surfactants. Ligand-stabilized AuNPs were selected as the NP model because of their well-defined chemical composition and structure, well-studied toxicity, and dispersibility as discrete structures without additional dispersal agents (that would cloud the interpretation of the data by producing a more complex mixture). The AuNPs herein were stabilized by covalently bound mercaptoethoxyethoxyethanol (MEEE) ligands, which have been shown to increase AuNP safety and biological circulation time.30,31 These AuNPs were tested in vivo using embryonic zebrafish as an animal model for rapid screening. Embryonic zebrafish were used to perform whole-animal investigations because they develop externally and a female is able to produce hundreds of eggs, thereby allowing for large sample sizes.37 The zebrafish assay is also time and material-efficient, only requiring a few milligrams of AuNPs to perform a rigorous broad test concentration range study in just a few days.38 We performed extensive testing of a range of surfactant concentrations, AuNP concentrations, and AuNP sizes, to observe how surfactant/AuNP mixtures affect zebrafish development. Inductively coupled plasma mass spectrometry (ICP-MS) was employed to analyze changes in biological uptake of the AuNPs in the presence of surfactant. We primarily focused on studying AuNPs with PS20, but also examined Polysorbate 80 (PS80) and sodium dodecyl sulfate (SDS) as potential alternatives for minimizing mixture synergies. All of the surfactants selected for study are recognized as safe by the U.S. Environmental Protection Agency,39 and approved for direct addition to foods by the U.S. Food and Drug Administration.40


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These particular surfactants were selected for their ubiquity in consumer products and high production volumes.41,42 AuNP and PS20 exhibit a synergistic toxicity in vivo Toxicity assays were performed by exposing zebrafish embryos (8 hours post fertilization (hpf)) to AuNPs and monitoring embryo development over the course of five days. Embryos were exposed to five sizes of AuNPs with core diameters ranging from 1.0 to 3.9 nm (sizes and distributions listed in Table S1). Each size was exposed at five concentrations ranging from 2.32 to 50 µg/mL, a concentration range that is typical for examining the biocompatibility of nanomaterials.43 Our previous work studying the toxicity of MEEE-AuNPs examined concentrations ranging 1-250 µg/mL and, although the particles are stable even at those high concentrations, we focused here on the lower concentrations that are closer to those used in applications. AuNPs were delivered to the embryos using one of two mechanisms, traditional hand-loading with micropipettes or digital dispensing with the BioPrinter. As shown in Figure 2, the results of the hand pipetting experiments (Figure 2b) were strikingly different than those obtained from the BioPrinter (Figure 2a). For example, 50 µg/mL of 1.0 nm AuNPs dispensed using the BioPrinter resulted in 88% mortality; while hand-loading of the same AuNPs only resulted in 3% mortality (background mortality for non-exposed animals is ≤10%). The only substantive difference between dispensing methods is that the BioPrinter requires a small amount of PS20 to break solution surface tension of aqueous samples. The stock sample in the printhead (containing 0.3% PS20 + AuNPs) is diluted 100X when dispensed into embryo-containing wells, resulting in a final well concentration of 0.003% PS20. This concentration of PS20 does not cause any mortality individually (see 0 µg/mL condition in Figure 2a). A schematic of the BioPrinter setup is included in the supporting information (Figure S1). 8 ACS Paragon Plus Environment

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In the toxicity profiles shown in Figure 2, each circle represents one embryo experiencing the endpoint of interest (mortality at 24 hpf in Figure 2, other endpoints are included Figure S2). Fisher’s exact test is used to determine if a null hypothesis is supported, if not, the Figure 2. Mortality profiles of 1.0 nm AuNPs at 24 hpf in embryonic zebrafish. Graphs display the number of zebrafish embryos affected under each condition (numbers also provided in Table 1, N=32 per exposure condition). The red data indicate statistically significant incidents. Embryos were exposed to AuNPs either by (a) digital dispensing using the BioPrinter in the presence of PS20 or (b) without PS20 (by hand-pipetting). The complete data sets with 22 endpoints are shown in Fig. S2.

treatment affected the incidence and that response is statistically significant. We indicate responses above this statistically significant threshold by coloring those data points red. Figure 2 shows that in the

absence of PS20, 1.0 nm AuNPs have low toxicity, which is consistent with what we31 and others44 have previously observed for polyethylene glycol coated AuNPs in this size range (1-4 nm). The significant toxicity observed when AuNPs are dispensed with the BioPrinter was a consistent trend for all sizes of AuNPs tested (Figure S3).


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Table 1 lists the toxicity caused

Table 1. Chemical influence on the incidence of mortality at 24 hpf (MO24) in embryonic zebrafish

by 1.0 nm AuNPs individually, PS20 [AuNP1.0 nm] (µg/mL) 2.3 5.0 10.7 23.2 50.0

[PS20] (% v/v) 0 0 0 0 0

MO24(%) 3 3 3 3 3

0 0 0 0

0.0003 0.003 0.03 0.3

3 0 13 81*

2.3 5.0 10.7 23.2 50 23.2 23.2 23.2 23.2

0.003 0.003 0.003 0.003 0.003 0.0003 0.003 0.03 0.3

0 16 22 56* 88* 9 53* 94* 100*

32 zebrafish embryos were exposed to each treatment. Asterisks indicate significance as determined by Fisher’s exact test, *p < 0.01. Background mortality for nonexposed controls is ≤10%.

individually, and mixtures of the two. The

AuNPs

insignificant,

have

low,

mortality

statistically through

50

µg/mL. PS20 has insignificant mortality until a concentration of 0.3% is reached. Comparing these individual toxicities to the results from initial studies using the BioPrinter for dispersal (where PS20 concentration is always 0.003% and AuNP concentration is varied, Figures S2 and S3), shows that the extent of mixture synergism

depends

on

AuNP

concentration. For AuNP1.0 nm, the onset of synergy occurs at 23.2 µg/mL of AuNPs. Thus, we used this concentration of AuNPs to assess the extent of mixture synergism when the amount of PS20 is varied. The synergistic effects of the AuNP/PS20 mixtures depend upon not only the concentration of AuNPs, but also the amount of PS20 in solution. When the AuNP concentration is held constant and 0.0003% PS20 is added, there is no increased mortality in the zebrafish (the 9% mortality observed is consistent with background incidence). Mixtures of AuNPs and either 0.003 or 0.03% PS20 exhibit significant mixture toxicity even though the components alone do not cause toxicity. Since the 0.3% PS20/AuNP mixture kills the whole zebrafish cohort, and


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0.3% PS20 induces toxicity on its own, it is not feasible to determine the extent of synergism under these conditions. Collectively, these data show that synergistic effects are modulated based upon chemical composition of the mixture; changing the amount of PS20 in the mixture can decrease mortality incidence from 94% to 9%, despite both concentrations of PS20 only causing background mortality when tested without AuNPs. The observed synergy, with respect to zebrafish mortality, suggests that there may be an interaction between the PS20 and the AuNPs. It is well established that the toxicities of NPs can be altered due to the free ions, released ligands, unpassivated NP surface, or the change in core45,46 that occurs when NPs dissolve or agglomerate in solution. Therefore, we examined whether PS20 was altering the structure of the AuNPs, which could lead to the observed synergy. PS20 and MEEE-AuNPs interact to form assemblies The types of structural changes that might occur include AuNPs agglomerating or forming assemblies with PS20, PS20 displacing MEEE ligands, or PS20 etching/dissolving the AuNP core. UV–vis was used to monitor the stability of the AuNP cores and their dispersion in the presence of PS20, for each core diameter. UV–vis is an informative technique for AuNPs in this size range because changes of just a few atoms in the core, either by dissolution or agglomeration, will result in a different optical signature.47,48 Comparing the absorption spectra of the AuNPs with and without PS20 shows no differences between the two, suggesting that PS20 is not destabilizing the AuNPs (representative results are shown in Figure S4, this stability was consistent for all five AuNP sizes). However, the smallest AuNPs studied herein (AuNP1.0 nm)

do not have a localized surface plasmon resonance peak so UV–vis may not detect small

changes in NP size or dispersity. Therefore, we collected small-angle X-ray scattering (SAXS) data on AuNP1.0 nm in the presence of PS20 to confirm the UV–vis results. The scattering patterns 11 ACS Paragon Plus Environment

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measured by SAXS depend upon NP size; dissolution of NPs leads to changes in the scattering pattern and/or a comparatively lower scattering intensity, while agglomeration will cause a Bragg peak at a low q value.48,49 The measured SAXS patterns showed that the AuNPs exist as a single size population and retain the same average core size after combination with PS20 (Figure S5). There is no evidence of NP agglomeration in the SAXS patterns. Because SAXS and UV– vis measurements are sensitive to small changes related to the NP cores at this lengthscale, we thought it was unlikely that the increased toxicity was the result of changes that were too small to detect by these techniques. The combined results from SAXS and UV–vis suggest that enhanced toxicity might be the result of interactions between the PS20 and AuNP ligand shell, as opposed to AuNP core dissolution or agglomeration. If PS20 interacts with the AuNP’s ligand shell in solution it is likely through weak, noncovalent interactions, resulting in an AuNP/PS20 assembly, analogous to protein corona formation. Techniques to investigate the formation and structure of such assemblies typically rely on diffusion-based measurement such as dynamic light scattering (DLS) or DiffusionOrdered Spectroscopy (DOSY) NMR.50 These techniques provide information about the diffusion coefficient of a species or assembly, which is related to hydrodynamic radius via the Stokes-Einstein equation, and the radius can be converted to a hydrodynamic diameter (Dh).50,51 For clarity, Figure S6 illustrates the structural difference between an AuNP Dh and a core diameter (Dcore). DOSY is a powerful tool for measuring hydrodynamic size and, as opposed to DLS, can be used to determine the chemical composition of different populations in the mixture. From the DOSY data it is possible to determine the Dh, relative proportion, and chemical make-up of each diffusing population.50

In our studies, it is beneficial that DOSY displays the protons 12 ACS Paragon Plus Environment

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composing each diffusion population because it enables differentiation between populations of AuNPs, PS20 micelles, and AuNP/PS20 assemblies. Although the benefits of DOSY seemed clear, we wanted to evaluate the appropriateness of the technique for this application. One challenge with DOSY is the need for sharp peaks in the 1D-NMR spectrum. This limits its use for many NP systems, because ligand peaks are characteristically broadened due to their reduced molecular tumbling.52 Fortunately, our smallest size of AuNPs (Dcore = 1.0 nm) rotate fast enough to have well-resolved peaks, thus allowing for tracking of the AuNP proton signals in the 2D spectra (Figure S7). A second concern we had was whether the calculated diffusion coefficients would be influenced by small differences in temperature and viscosity, two important variables in the Stokes Einstein equation.50,51 We examined the influence of these variables by computing Dh with the largest variation expected, as well as testing the viscosity changes experimentally with a mannose standard. The amount of variation calculated was smaller than the experimental error of the measurements in each case (see SI for details). The last challenge we anticipated for the DOSY experiment was obtaining sufficient signal/noise for the MEEE-AuNP peaks of interest. Since the purpose of this study was to correlate structural changes with toxicity profiles, it was necessary to mimic the same parameters used in toxicity assays. Therefore, increasing the AuNP concentration was not a feasible method of increasing the signal. Increasing the number of scans could increase signal, but would limit our ability to study what we anticipated would be a dynamic system once PS20 was added. To determine the best experimental parameters for obtaining accurate data from the relatively small MEEE-AuNP peaks, we first performed DOSY studies on AuNP1.0 nm alone in D2O. Using this sample we determined a set of experimental parameters53 (e.g., number of scans 13 ACS Paragon Plus Environment

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(NS), range of gradient strength (g), diffusion time (∆) , and diffusion gradient length (δ)) that provided a Dh value consistent with what is expected based upon a 1.0 nm core and MEEE ligands. Despite our initial concerns that detecting the whole decay function properly would require multiple hours of acquisition, we identified experimental parameters that allow for a 35 min acquisition time with results that show no difference in the MEEE-AuNP 2D spectrum compared to a 4.5 hr acquisition time. The 2D DOSY spectrum for pure MEEE-AuNPs is shown in Figure 3 The horizontal spectrum displays the

1

H-NMR peaks and the vertical spectrum displays the diffusion

populations, which are just residual HDO and one population of AuNPs in this case. The peak at 4.98 ppm was used as the unique signal for tracking AuNPs in solution with PS20, and a second AuNP peak, at 3.26 ppm was used to confirm that the peak at 4.98 was not affected by the HDO signal that results from proton impurities in the D2O solvent. The 3.26 and 4.98 ppm AuNP peaks do not overlap with PS20 signals (Figure S7), making it possible to attribute the measured diffusion coefficients to

Figure 3. 2D DOSY spectrum for MEEE-AuNP1.0 nm in D2O. The peak at 4.98 ppm was used to track the size of the AuNPs throughout experiments, and confirmed by the peak at 3.26 ppm.

the AuNPs, not PS20 micelles. The maximum of the MEEE-AuNP diffusion population is 1.95 x 10-10 m2s-1, corresponding to a Dh of 2.5 +/0.1 nm (reported uncertainty is the experimental variation in the peak maximum, as determined by triplicate measurements).


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DOSY NMR was used to monitor the evolution of AuNPs over time in the presence of varying concentrations of PS20. Samples contained 1000 µg/mL of AuNPs to mimic BioPrinter stock solution conditions (i.e., conditions the NPs were exposed to prior to dispersal with zebrafish, set-up details are included in Figure S1). Importantly, both DOSY and SAXS data demonstrated that even at this relatively high concentration of 1000 µg/mL, AuNPs remain a single, non-aggregated, population in the absence of PS20. Samples were monitored for 18 hrs: 18 hrs is the amount of time between the initiation of embryo exposure and the first endpoint assessment. At the start of each study samples had 0.3, 0.03, 0.003 or 0.0003% (v/v) PS20 added. The AuNP/PS20 mixtures were monitored closely for the first two hours, as that is when most of the changes in AuNP size occurred. A control sample, containing only AuNPs and no PS20, remained unchanged over the course of 18 hours. New AuNP-containing species with lower diffusion coefficients appear almost immediately after PS20 addition, suggesting rapid assembly of PS20 onto the AuNPs. Because each of these populations contains proton signals from both the MEEEAuNPs and the PS20, the data provide further

evidence

for

MEEE-

AuNP/PS20 assemblies. The diffusion coefficients for both the AuNPs and PS20 micelles are different in the mixture than when measured as single components (Figures S8 and S9), suggesting Figure 4. Summary of DOSY data tracking MEEEAuNP1.0 nm growth in the presence of varying concentrations PS20 over an 18 hour period. Error bars (+/- 0.1 nm) represent the experimental variation in the MEEE-AuNP size, as determined by triplicate measurements ACS Paragon Environment on a sample of pure AuNPs. In the absence of PS20,Plus MEEEAuNP1.0 nm have the same response as the 0.0003% PS20 trendline, exhibiting no change in Dh.

that the solvation sphere of

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both the PS20 and the AuNPs are affected by one another. We monitored the interactions between PS20 and MEEE-AuNPs by collecting DOSY spectra over 18 hours and determining the maximum intensity for each AuNP/PS20 diffusion population. The diffusion coefficients from the slowest moving assembly at each time point were converted to a Dh and plotted in Figure 4. It is important to note that as the concentration of PS20 increased, so did the distribution of AuNP/PS20 assembly sizes. In all cases the largest assembly size was tracked because that population is expected to have toxicity that deviates the most from unassembled AuNPs. The data showed that the diffusion coefficients and relative proportions of the AuNP/PS20 assemblies are dynamic for the first two hours of mixing, but tend to stabilize for the remaining 16 hours of the study (representative DOSY spectra are included in Figures S10S13). Figure 4 shows that the MEEE-AuNP/PS20 assembly size depends on the concentration of PS20. Only the smallest concentration of PS20, at 0.0003%, did not induce a change in AuNP size. All other mixtures (>0.0003% PS20) formed assemblies within 20 minutes. Comparing the assemblies at 18 hours to the initial MEEE-AuNPs reveals that 0.3% PS20 caused 3.4 nm of growth in Dh, whereas 0.03% and 0.003% PS20 resulted in 1.8 nm and 0.5 nm growth, respectively. The size changes observed are consistent with a model where the PS20 forms a submonolayer or interdigitates into the covalently attached MEEE ligand shell. The data do not suggest that PS20 is forming a complete monolayer around the MEEE, because those particles would include sizes between 7 and 16 nm depending upon the structure and conformation of PS20 (see SI for calculations).


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Figure 5 displays the mortality data as a function of the percent PS20 added (from Table 1), along with the assembly size data determined by DOSY. The influence that a given amount of PS20 has on AuNP size is labeled with the relative increase in hydrodynamic Figure 5. Concentration of PS20 versus zebrafish mortality. Solid markers are AuNPs (23.2 µg/mL) + PS20. Labels next to the solid markers indicate the influence of PS20 on the assembly size (all AuNPs began with Dh = 2.5 nm). Hollow markers show the toxicity due to PS20 individually. Asterisks indicate significance as determined by Fisher’s exact test, *p < 0.01.

diameter (∆Dh) next to each data point. The plot shows that the formation of assemblies coincides with the onset of mixture synergism.

Further, the figure

demonstrates that AuNP/PS20 assemblies have different biocompatibility than the individual components, and this is particularly evident for the middle points on the graph (0.003% and 0.03% PS20). At these concentrations of PS20, the toxicity of the mixture is amplified by over 3fold compared to that of the individual components. Examining potential mechanisms for synergistic toxicity There are multiple mechanisms that could render PS20/MEEE-AuNPs more toxic than the sum of their components. One hypothesis for the observed toxicity is that the oligo-ethylene glycol coated AuNPs serve as a template for assembly of the PS20, and that the surfaces of these PS20 assemblies are responsible for increased toxicity. Alternatively, PS20, through interaction with the AuNP, might render the NPs themselves more toxic. We examined nanostructures that were similar to the 1 nm AuNPs, but lacking a metal core, so as to isolate the possible effects of the surface coating and the metal core. We aimed to find a metal-free structure with 17 ACS Paragon Plus Environment

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approximately the same dimensions and surface chemistry as our AuNPs. A commercially available dendrimer containing hydroxyl-terminated polyethylene glycol ligands, similar in length to the MEEE ligands, met these criteria (structures in Figure S14). The measured Dh for the dendrimer was 1.5 nm (compared to 2.5 nm for the AuNPs). The dendrimers were then evaluated, alone and in combination with PS20, in zebrafish embryos, employing the same conditions as the manually-dispensed AuNP assays. The isolated dendrimers did not cause any developmental toxicity and mixtures of the dendrimers with PS20 produced only additive effects (Figure S15). Further, DOSY NMR studies show that the dendrimers and PS20 form assemblies (Figures S16 and S17) like those found for the AuNPs with PS20. The formation of assemblies, but lack of synergism for these mixtures, suggests that PS20-coated oligo-ethylene glycol nanostructures are not the direct cause of the synergism, and implies that the mechanism of toxicity is not due to MEEE-AuNPs enhancing the delivery of PS20. Instead, the PS20/AuNP assemblies may have synergistic toxicity because of differences in AuNP bioavailability. To evaluate whether MEEE-AuNP/PS20 assemblies have enhanced uptake compared to AuNPs alone, we used inductively coupled plasma mass spectrometry (ICP-MS) to measure the mass of gold in zebrafish (n=3, 4 biological replicates) in the presence and absence of PS20 (Figure 6). The results showed that in the presence of PS20, 44% more gold enters the embryos. While this clearly demonstrates that PS20 Figure 6. Tissue concentration after exposure to gold, as determined by ICP-MS.

increases the gold tissue burden, an


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increase in the dose of AuNPs, alone, should not have increased toxicity at these concentrations based upon the data shown in Table 1 (in Table 1, both 23 µg/mL and 50 µg/mL of AuNPs cause only 3% toxicity). Instead, the 53% toxicity that is observed for the mixture of 23 µg/mL AuNPs and 0.003% PS20 suggests that the AuNP/PS20 assemblies are more toxic structures on a molar basis. Although the increased uptake of the assemblies (compared to the AuNPs alone) exacerbates the synergism, these results suggest that the assemblies have a different mechanism of toxicity than the AuNPs or PS20 alone. In previous work,54,55 we were able to use behavioral and developmental endpoints to assess differences in the mechanisms of toxicity. For the materials studied herein, however, this was prohibited by the high level of mortality caused by the mixtures and the low level of effects caused by the individual components (see Figure S2). Other common surfactants have synergistic interactions with MEEE-AuNPs We evaluated two other common surfactants, Polysorbate 80 (PS80) and sodium dodecyl sulfate (SDS) (Figure 7), to examine whether the synergistic toxicity found in the PS20/AuNP mixture might be a more general phenomenon. PS80 was selected to compare a surfactant chemically similar to PS20 but with increased hydrophobicity, and SDS was examined to compare an ionic surfactant to the nonionic polysorbates. Both PS20 and PS80 are derived from polyethoxylated sorbitan and a hydrophobic acid; in PS20 the hydrophobic chain is 11 carbons long and saturated, whereas in PS80 the hydrophobic chain is 17 carbons long and contains an internal double bond. In contrast, SDS is anionic, and consists of a 12-carbon alkane chain attached to a sulfate head group.


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In the presence of MEEE-AuNPs, the toxicity profile for PS80 is similar to that of PS20, although the magnitude of the synergistic effects is somewhat lower (Figure S18). For example, for 0.03% PS80/AuNPs at 24 hpf there is a measurable synergy, whereas

a

measurable

synergy

is

observed down to 0.003% in the case of PS20. The similarity between mixture effects for PS80 and PS20 might be Figure 7. Structures of additional surfactants studied in vivo. Developmental endpoints are shown in the SI.

predicted

based

on

their

similar

structures. The reduction in mortality rates in the case of PS80 may be due to structural differences between PS tails, changing the interaction with the MEEE ligands. In the presence of the anionic surfactant (SDS) at 24 hpf the toxicity is additive and essentially no synergy is observed; however, by day five, (see Figure S18) there is a notable synergistic increase in toxicity for the samples containing MEEE-AuNPs and either 0.03 or 0.003% SDS. The delayed onset of the synergistic effects suggests that the mechanism of synergy is different for SDS than with the Polysorbate surfactants. These results demonstrate that surfactants, as a class of compounds, may exhibit synergistic increases in toxicity with NPs, but the magnitude of these effects and the mechanisms causing them depend upon the surfactant structure.


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Conclusions In vivo studies were performed with five MEEE-AuNP sizes in the presence of Polysorbate 20 (PS20). Although the AuNPs and PS20 both exhibit low toxicity alone, any concentration of PS20 sufficiently high to produce an assembly with the AuNPs resulted in significantly increased toxicity for the mixture. Synergistic toxicity at low concentrations, 23 µg/mL of AuNPs and as low 0.003% (v/v) of PS20 is especially surprising because synergies generally occur at higher concentrations.13 NMR studies confirmed that PS20 assembles onto the periphery of the AuNPs under the biological exposure conditions. Studies on dendrimers with similar dimensions to the AuNPs show that they, too, form assemblies with PS20, but those assemblies do not cause increased toxicity. Those results suggest that PS20 increases the toxicity of the AuNPs, as opposed to the NPs inducing a toxic assembly of PS20. ICP-MS studies show that PS20 increases the uptake of AuNPs in the organisms compared to AuNPs alone. However, these differences in uptake do not account the observed synergy, suggesting that the assemblies are also more toxic per mole than the individual components. Lastly, we studied other surfactants and found that the AuNP/surfactant synergy is not exclusive to PS20; synergistic toxicity also occurred with Polysorbate 80 and sodium dodecyl sulfate. Our findings that the presence of surfactants can increase the hazards of a NP mixture are important for the nanotechnology community. Surfactants are often added to NP solutions to disperse nanomaterials or enhance the stability of dispersions,34 reduce the formation of free radicals,56 and improve the texture of cosmetic formulations.11 For example, NPs in sunscreens are often coated with dimethicone to decrease agglomergation and increase photostability,56 and NP toxicity studies sometimes employ PS80 in the media to promote solution dispersion.34,57 Moreover, when NPs enter surface waters or sewage systems they are likely to come in contact 21 ACS Paragon Plus Environment

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with surfactants, and understanding the conditions that promote these types of NP transformations will be important for evaluating environmental risks.58,59 For nearly a century the rarity of documented synergies, along with the combinatorial nature of mixture toxicology, have motivated a standard practice of treating mixture toxicity as the sum of individual components.12,13,20 Based upon our findings, the synergistic role of surfactants on NP toxicity should be considered in the design of investigations or products. It is not safe to assume that a mixture of “non-toxic” ingredients, such as surfactants and NPs, will produce a benign mixture. In addition, these findings should be considered when interpreting previous studies that have employed NP/surfactant mixtures, such as those assessing nanomaterial safety or the formulation of cosmetic formulations, where the outcomes may have been influenced by the surfactants used. Given the widespread use of surfactants both industrially and academically, it is important to identify strategies for selecting safe NP/surfactant combinations. By combining toxicological analysis with a molecular design approach,60 NP/surfactant mixtures can be formulated with the desired properties without the introduction of undesired hazard. Our studies suggest this is possible because even among chemically similar Polysorbates, there are differences in synergistic effects. It will be important to identify or design alternative chemicals that do not result in a loss of function or performance.61 Recent advancements in high-throughput technologies and computing may ease the process of identifying mixtures that deviate from additive dosimetry models.62–64 To enable deliberate chemical selection, there first needs to be studies conducted that form foundational design rules. Our recommendations for maximizing the impact of these studies are to: select NP mixtures with known composition and a minimal number of components, prioritize mixtures by the relevance of the components, and compare chemicals of similar function against one another.


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Methods Chemicals All materials were used as received. HAuCl4･H2O (99.9%) was purchased from Strem, 2-[2-(2chloroethoxy)ethoxy]ethanol (99%) was from TCI America, sodium thiosulfate (anhydrous) was from Mallinckrodt, pronase was from Roche, sodium dodecyl sulfate was from J.T. Baker, thiourea was from Aldrich, and Tween 20 (i.e., Polysorbate 20) was from Acros. Tween 80 (i.e., Polysorbate 80), triphenylphosphine and glycerol ethoxylate (average Mn ~1,000) were from Sigma-Aldrich. Deuterium oxide was purchased from Cambridge Isotope Laboratories and stored in the dark. Nanopure water (18.2 MΩ·cm resistivity) was obtained using a Barnstead Nanopure filtration system and used for all syntheses and assays. Preparation and Characterization of MEEE-AuNPs Mercaptoethoxyethoxyethanol (MEEE) is synthesized via a substitution reaction with the chlorinated precursor, 2-[2-(2-chloroethoxy)ethoxy]ethanol, and thiourea.65 MEEE-AuNPs that are 1.0 nm in metal-core diameter ((Dcore) as measured by SAXS), and 2.5 nm in hydrodynamic diameter ((Dh) as measured by DOSY) were synthesized according to previously reported methods.31 Briefly, phosphine-stabilized AuNPs were prepared as intermediates and then underwent a biphasic ligand-exchange with MEEE.66 AuNP1.0 nm were purified via size-exclusion chromatography, as confirmed by 1H-NMR. The 1H-NMR spectrum was collected in 500 µL of D2O (residual HDO referenced at 4.79 ppm) on a Bruker Avance III-HD 600 MHz Spectrometer. The other AuNP sets with core diameters ranging from 2.8 to 3.9 nm (sizes and distributions are detailed in Table S1) were synthesized using a mesofluidic reactor where the pH of the gold salt 23 ACS Paragon Plus Environment

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precursor was varied to control core size,67 and then purified using diafiltration.68 Following purification, all sets of AuNPs underwent lyophilization and were stored as dried powders. The AuNP core size and distribution was determined by SAXS and corroborated by TEM to confirm size and shape. SAXS measurements were performed on an Anton Paar SAXSess instrument in in-line collimation mode. SAXS measures AuNP core size in solution, therefore it was possible to measure changes to gold core in the presence of Polysorbate 20 (PS20). Samples for analysis of PS20 effects were prepared at 1000 µg/mL of AuNP1.0

nm,

and 50 scans were

collected at 70-second exposure times. In the SAXSquant software, the raw NP scattering pattern was background and dark subtracted and desmeared using the beam profile generated by the transmitted beam (patterns are shown in Figure S5). TEM confirmed that the sizes obtained by SAXS were valid and that the particles were indeed spherical. TEM measurements were collected on a FEI Titan (S)TEM equipped with an Cs aberration corrector at 300kV. TEM samples were prepared by floating a copper grid on a dilute solution of AuNPs for ~2 mins, then floating the grid on water for ~2 mins, and then wicking away excess liquid with a Kimwipe. Select dark-field and bright-field images are provided in the SI (Figure S19). Dispersion stability of the AuNPs was assessed with UV–vis. Spectra were collected using an Ocean Optics USB2000 spectrometer with samples in a quartz cuvette (1 cm path length). There was no change in the absorbance of the particles (both with and without PS20) within the time frame and concentrations of the toxicity assays (Figure S4). The dispersion stability was also assessed using SAXS and the average core size was unchanged over 18 hrs in the presence of PS20 (Figure S5b). Zebrafish Care and Husbandry


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Tropical 5D adult zebrafish were housed at Oregon State University Sinnhuber Aquatic Research Laboratory on a recirculating system at 28ºC on a 14h light:10h dark photoperiod. Spawning funnels were placed into the tank the night prior and embryos were collected and staged.69 The chorion was enzymatically removed using pronase (90 µL of 25.3 units/µl) at 4 hpf using a custom built dechorionator.70 Adult care and reproductive techniques were conducted according to the Institutional Animal Care and Use Committee protocols at Oregon State University (OSU). Nanoparticle delivery For these studies, the MEEE-AuNPs were stored as dried powder in various masses. To standardize all solutions made, each sample used was brought to a stock concentration of 1000 µg/mL by adding an appropriate volume of ultrapure water. Two techniques for delivering the AuNPs were employed: manual pipetting, or the use of Hewlett-Packard D300 digital dispenser (referred to as the BioPrinter).71 The BioPrinter uses inkjet technology to digitally dispense chemicals. This technology eliminates the need for serial dilution and increases accuracy.62 For manual hand pipetting, concentrated stock of AuNPs (1000 µg/mL) were transferred into sterile glass tubes, and brought to the specified concentrations using ultrapure water. Five-point serial dilutions of the AuNPs were made ranging from 2.32 to 50 µg/mL. Developmental toxicity screen For manual hand pipetting experiments, dechorionated embryos were manually loaded into the wells prefilled with 100 µL of the various concentrations of AuNPs and/or PS20. The experiments using the BioPrinter required embryos to be loaded into individual wells prefilled with 100 µL of ultra pure water. After the embryos were loaded, a stock concentration of AuNPs


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(1000 µg/mL) with 0.3% PS20 was dispensed using the BioPrinter to achieve the desired test concentration of AuNPs and 0.003% PS20 in one step (BioPrinter set-up and schematic in Figure S1). After dispensing of the chemicals, to ensure uniform mixing, the plates were moved to a temperature-controlled room with a custom modified shaker and shook overnight.62 For each AuNP size, six concentrations (a control without any AuNPs, and five concentrations of AuNPs) were assessed, with a total of 32 animals exposed to each concentration. During the experiments, all plates were covered in parafilm and foil to prevent evaporation and protect against potential AuNP degradation. In manual hand pipetting experiments where one concentration of the AuNP was used with a varying percentage of PS20, a 50 mL solution of AuNPs at 23.2 µg/mL was made. PS20 was diluted and added to 23.2 µg/mL AuNP aliquots to make 3 separate solutions with final concentrations of 0.3, 0.03, 0.003 and 0.0003% PS20. At 24 and 120 hpf, 22 morphological endpoints were evaluated and collected in a custom laboratory information management system, the Zebrafish Acquisition and Analysis Program (ZAAP).62,72 These 22 endpoints include mortality, developmental progression, and eye/jaw defects. The control group (non-exposed animals) were assessed to determine if the number of affected subjects is below 20% for all measured endpoints combined (and less than 10% for mortality alone) at 120 hpf. These thresholds were established statistically from historical data and based upon various large chemical screens. Because developmental testing involves numerous complex series of steps, there is variability in background incidence between experiments. To differentiate background from real signal, statistical analysis was performed as previously described by Truong et al.73 A representative figure displaying the data from all endpoints is included in the supplemental information (Figure S2). 26 ACS Paragon Plus Environment

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2D NMR spectroscopy on AuNPs plus surfactant The diffusion NMR experiments were performed at an AuNP concentration of 1000 µg/mL. Although toxicity studies only go through 50 µg/mL of AuNPs, we chose to use 1000 µg/mL, as this is the initial concentration of AuNPs present in the BioPrinter incubator prior to dilution in the embryos. The NMR data suggest that the PS20 and AuNPs associate almost immediately upon addition, therefore changes to the AuNPs will happen during the incubation period and we wanted NMR studies to reflect those changes. Stock solutions of PS20 in D2O were made such that 8 µL of diluted PS20 was added to 492 µL of AuNPs in D2O for a total volume of 500 µL, where the final PS20 concentrations ranged 0.3-0.0003% by volume. All 1H NMR experiments were performed on a Bruker Avance III-HD 600 MHz NMR Spectrometer and acquired at 298.2 K. Data was collected in the TopSpin software and processed in MestReNova version 10.0.2. During acquisition, each DOSY sample first had a 1D spectrum collected with 4 dummy scans and 32 scans. The line broadening was set to 1.00 and the acquisition time to 1 s, then the 1D spectrum was solvent suppressed. The pulse sequence was set to ledbpgppr2s with diffusion time (∆) equal to 0.1 s and diffusion gradient length (δ) equal to 3400 ms, 4 dummy scans and 32 scans, and a varying gradient power from 5-55% collected over 30 spectra. This gradient range was determined to be appropriate for properly sampling the data points along the whole decay curve from the MEEE-AuNP peaks of interest.53 Each 2D NMR spectra took about 35 minutes of acquisition time. All spectra were collected in D2O and referenced to residual HDO at 4.79 ppm. It is important to note that since quantification of DOSY data is relatively uncommon, we first validated this method using a mannose standard. Additional information about processing the DOSY data is provided in the SI.


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Gold tissue concentration Zebrafish embryos that were 8 hpf were statically exposed to 0 (ultrapure water), 0.003% PS20, 23.2 µg/mL AuNPs, and 23.2 µg/mL AuNPs + 0.003% PS20. It was not feasible to conduct these experiments at 50 µg/mL exposures because most of the population dies within 18 hpf, making the exposure times uncertain. The populations were sampled at 24 hpf to quantify tissue concentration of gold as previously published,74 with a modification of the wash step. Embryos were washed in the 96-well plate by adding 100 µL of ultrapure water, and quickly removed using a Rainin liquidator. This was repeated 10 times. Three embryos were transferred to 14 mL round bottom plastic centrifuge tubes and excess water was removed. There were four biological replicates per exposure group, with a biological replicate made up of three animals. Collected samples were stored in -80 °C until time to sample. Using a Thermo X-Series II Inductively Coupled Plasma-Mass Spectrometer (ICP-MS), gold concentrations were measured in the samples. For each sample, internal standards (indium, rhenium and bismuth) were added with a final concentration of 2 ppb in a total volume of 5 mL. A 5 point standard curve was created using a purchased gold standard (0.01, 0.1, 1, 5, and 10 ppb) and had a R2 value of 0.997. Acknowledgments A. Ginzburg acknowledges support from the NSF Graduate Research Fellowship Program under Grant DGE-1309047. This research was partially supported by NSF grant #1512755. The NMR instrument facility at the University of Oregon is supported by the NSF (CHE-1427987). Dr. M. Strain and J. Barry provided invaluable guidance on performing and processing DOSY experiments. We thank Dr. T. Zaikova for insightful scientific discussions and the instrument managers in CAMCOR for their expertise.


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Supporting Information. SAXS patterns and data for AuNP size and distribution, TEM images, UV-Vis spectra of AuNPs with and without PS20, description of DOSY experiment and data processing, 1H-NMR spectra of MEEE-AuNPs and PS20, 2D NMR spectra of AuNPs and PS20 individually, a series of 2D NMR spectra for an AuNP/PS20 mixture over 18 hours, calculations of mixture viscosity, determination of AuNP/PS20 assembly structure, schematic of BioPrinter dispersal method, and zebrafish toxicity profiles. References (1)

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TOC Figure


Synergistic Toxicity Produced by Mixtures of Biocompatible Gold

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